I too have used gundam markers in the past. I've also used mechanical pencils, technical pens (rapidograph), etc... in attempts to find the best method for panel lining.... By far, the easiest, and least amount of skills involved is the following:
Wash technique <-- link is to another forum, but there are pictures and the tutorial is fairly extensive so to rewrite it here would just be laborious.(plus, it was written by me) So long as the rules for the technique are followed, this is the absolute easiest method for panel lining. (in my opinion, of course)